1. Field of the Invention
The present invention relates to an ink jet recording head of an ink jet recorder for recording characters or images by discharging ink droplets toward a recording medium, and more particularly to such an ink jet recording head which uses a piezoelectric element as an electro-mechanical transducer for discharging the ink droplets.
2. Related Background Art
An ink jet recording head of this type has been proposed by U.S. Pat. No. 3,683,212 by Gould Inc. In this head, as shown in FIG. 1A, a cylindrical piezoelectric element 1 is polarized on its inner circumference and outer circumference, and a positive pulse voltage shown in FIG. 2 generated in response to an input signal by a pulse generator 2 is applied in the same direction as the polarization so that an impact stress is applied to the piezoelectric element 1 to cause a nozzle 4 to discharge an ink as an ink droplet 3 stored in the piezoelectric element. Arrows in FIG. 1B show the directions of polarization.
After the ink droplet has been discharged, the ink surface at the end of the nozzle 4 is retracted but the surface tension of the ink at the nozzle acts to increase a radius of curvature of the ink surface. Thus, the ink is supplied into the nozzle 4 through an ink supply path 5.
However, since a sectional shape of the piezoelectric element 1 of the prior art head is concentric and circular, when a plurality of heads are arranged in parallelly to form a multi-nozzle structure in order to increase the print speed or allow multi-color printing, a section of the entire head assembly includes a series of circles as shown in FIG. 3. Thus, when the diameter of the piezoelectric element 1 is represented by d, a pitch P1 between the centers of the nozzles 4 must be larger than d (P1&gt;d). Thus, the head assembly is of large size for the multi-nozzle ink jet recording head.
On the other hand, in a piezoelectric element of such shape, if electrodes are arranged on the entire surfaces of the inner and outer circumferences of the cylinder and a voltage is applied thereacross, the cylindrical element is deformed in the inner circumference as shown in FIG. 4 by a reverse-piezoelectric effect. A solid line shows the original inner circumference of the piezoelectric element 1 and a broken line shows the inner circumference after the application of the voltage, and .DELTA.l and .DELTA.d represent variances of length and diameter. Thus, the length l changes to l+.DELTA.l while the diameter d changes to d-.DELTA.d. In the ink jet recording head of this type, since the electrodes of the piezoelectric element 1 are arranged over the entire surfaces of the inner circumference and the outer circumference of the cylinder, the manufacture of such a device is not easy. A piezoelectric vibration mode is a combination of a longitudinal vibration and a lateral vibration and the cylindrical shape is maintained after the deformation. Accordingly, a sufficient electro-mechanical transducing efficiency is not attained.